1. Field of the invention:
This invention relates to a semiconductor laser which has diffraction gratings therein allowing the selection of a wavelength. More particularly, it relates to a GaInAsP/InP distributed feedback semiconductor laser which is useful as a light source in optical communication systems.
2. Description of the prior art:
Since GaInAsP/InP double-heterostructure semiconductor lasers having an InP substrate attain laser oscillation at a wavelength ranging from 1.0 to 1.6 .mu.m, they are useful as a light source in optical communication systems incorporating optical fibers. Especially, distributed feedback semiconductor lasers (DFB lasers) which have a diffraction grating with a periodic corrugation on or near the active regon attain laser-oscillation operation at a single wavelength even when the oscillation wavelength is being modulated, and accordingly they are optimal as a light source for future optical communications sytems. DFB lasers are fabricated as follows: As shown in FIG. 2(A), on the surface of an n-InP substrate 1, a diffraction grating 2 is formed, first. Then, on the side of the diffraction grating 2 of the InP substrate 1, an n-GaInAsP optical guiding layer 3, a GaInAsP active layer 4, a p-InP cladding layer 5 and a p-GaInAsP cap layer 6 are successively grown by liquid phase epitaxy, resulting in the DFB semiconductor laser having a double-heterostructure multi-layered crystal for laser oscillation as shown in FIG. 2(B). The bandgap energy of the optical guiding layer 3 is greater than that of the active layer 4, so that light can be propagated into a region extending from the active layer 4 to the optical guiding layer 3. On the other hand, carriers injected into the semiconductor laser are confined within the active layer 4. That is, the above-mentioned DFB semiconductor laser has a socalled SCH (separate confinement heterostructure) structure. This DFB semiconductor laser is disadvantageous in that the Fabry-Perot resonantor which is constituted by both facets resulting from the cleavage of the wafer produced by the above-mentioned process, achieves laser oscillation not only in the DFB mode but also in the F-P mode so that the laser oscillation becomes unstable. In order to prevent laser oscillation in the F-P mode, one of the facets is slanted in the resonance direction by means of a slanting means. Moreover, the above-mentioned DFB semiconductor laser is disadvantageous in that even though the sloped facet is formed to prevent the reflection of light therefrom to thereby suppress the laser oscillation in the F-P mode, there is a possibility of laser oscillation in two DFB modes depending upon the reflectivity of the other facet and the phase of laser light therefrom since the other facet has an influence on the DFB mode. The threshold gain .alpha. of laser oscillation in the DFB mode with the reflection of light from the other facet of the resonator is represented as follows: EQU (.tau.L).sup.2 +(.kappa.L).sup.2 sin h.sup.2 (.tau.L) (1-Rl) +2i.kappa.LRl.tau.L sin h(.tau.L)cos h (.tau.L)=0 EQU .tau.=(.alpha.-i.DELTA..beta.).sup.2 +.kappa..sup.2
wherein L is the length of the laser device, .kappa. is the coupling coefficient, Rl is the reflectivity of the facet which is 0.565exp(i.theta.) (wherein .theta.=.pi.), and .DELTA..beta. is the deviation of a propagation constant from the Bragg wavelength.
FIG. 3 shows the dependence of .alpha.L on .alpha..beta.L which are obtainable from the above-mentioned equations. Given that .kappa. is 40 cm.sup.-1 and L is 500 .mu.m, .kappa.L is 2.0 and these two modes have the same threshold gain in both side of the Bragg wavelength, resulting in laser oscillation in the two modes, the spectra of which is shown in FIG. 4, indicating that when laser oscillation is achieved in the two modes, laser oscillation in a single longitudinal mode cannot be obtained notwithstanding that the purpose of use of such a DFB laser is to obtain laser oscillation in a single longitudinal mode.
As mentioned above, depending upon the combination of the reflectivity of a facet of DFB lasers, the phase of laser light therefrom and the coupling coefficient thereof, DFB lasers do not oscillate in a single longitudinal mode.